The invention generally relates to a supply voltage sequencing circuit.
A typical computer system includes a power supply that provides and regulates various supply voltages that are used by and power the components of the computer system. As examples, the computer system may provide and regulate supply voltages for 5 volt (V), 3.3 V, 2.5 V, 1.8 V and 1.5V power planes (also called rails or voltage supply lines) of the computer system.
The power supply does not instantly provide the supply voltages during startup of the computer system. Rather the power supply generally has a transient response that establishes a delay in initially providing, or bringing up, the supply voltages when the computer system is turned on. Furthermore, the power supply may provide some of the supply voltages before others. For example, the power supply may generate a 3.3 V supply voltage for one power plane and further convert the 3.3 V supply voltage to a 1.8 V supply voltage for another power plane. In this manner, there may be a significant delay between when the power supply brings up the 3.3 V supply voltage (that comes up first) and when the power supply brings up the 1.8 V supply voltage. For example, this delay may be attributable to the power supply using a control voltage to convert the 3.3 V supply voltage to the 1.8 V supply voltage, and the power supply may have to wait on the control voltage to come up before the conversion of the 3.3 V supply voltage into the 1.8 V supply voltage takes place. Delays may also exist in the timing in which the power supply removes, or brings down, the supply voltages when the computer system powers down.
A component of the computer system may have a requirement that two power supply voltages that are received by the component must remain within a predefined voltage difference, even during the startup and power down of the computer system. Otherwise, damage to the component may occur.
One possible solution to this problem is to use a converter that does not use a control voltage to convert one supply voltage into another supply voltage. However, such a converter typically is substantially more expensive to make than a converter that uses the control voltage. Another solution may be to use a string of serially coupled diodes to create a voltage drop from one supply voltage to generate another supply voltage. However, a drawback of this solution is that the forward voltage drop of the diodes must be closely controlled in an environment where a variety of different currents may be drawn. Otherwise, the voltage difference specification may be exceeded. Another solution may be to couple the sensitive components to the plane that the supply voltage that appears at an earlier time only after sensing the presence of the supply voltage that appears at a later time. However, switching circuits to selectively couple the components to the power plane must have a low resistance, thereby making this design difficult to implement.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.